Note: What follows is another post in an on-going series for developers who are interested in learning to write applications for the iPhone. The entire series can be found here:iPhone Developer .

The code for creating new instances of a class generally looks as follows:

SomeClass *ptr = [[SomeClass alloc] init];

In this case, we send a message (alloc) to the recevier (SomeClass) to create a new instance; the object returned from this message, then sends a message (init) to initialize instance variables within the class.

I briefly talked about the importance of the order in which objects are allocated and initialized. You can read more about that here: Memory Management in Objective-C . In this post, I want to introduce how to override the init method of the NSObject class, more specifically, how to work with multiple initializers.

In the init method of the NSObject class, no initialization takes place, it simply returns self .

The basic format of a method to override the initializer looks as follows:

It’s important to call the super init method first to ensure that instance variables up the inheritance hierarchy are initialized in the proper order (from top to bottom). Also, if your initialize code fails, you need to return nil from the overriden init method. Finally, return a reference to self as shown above.

So let’s assume we have a block of code as follows:

SomeClass *ptr =[[SomeClass alloc] init];
[ptr setStr:@"Testing"];

Here we initialize a new instanc of SomeClass, and follow this with a call to set an instance variable to the specified string (@”Testing). One common means to accomplish this is to create a new initializer in which we pass in the parameter (the string in this case) as part of the original creation of the object. For example:

SomeClass *ptr =[[SomeClass alloc] initWithStr:@"Testing"];

We can also take this one step further. Let’s say that we also wanted to initialize an instance variable that was a pointer to a date object (NSDate *). In that case, we might want an additional initilizer that looks as follows:

It quite common for classes to have more than one initializer for creating new objects, allowing variations as shown above. This also implies that the initialization methods need to work in harmony. Designated initializers are the means to achieve this harmonious state.

Designated Initializers
When working with a class that has more than one initialization method (as shown above), it’s important that one of the initializers drives the whole process. Put another way, only one of the initializer methods does the actual work of calling the super class initializer and initializing the instance variables of the object.

This process is actually quite simple. Let’s assume we have a class implementation as follows:

The way this should work, is that a call to the initializer init should call initWithStr . A call to the initializer initWithStr should call initWithStrAndDate . Following this process, all the actual initialization work occurs in initWithStrAndDate . This method (the one that does the work) is known as the designated initializer .

A general rule of thumb (although not always the case) is that the designated initializer is the initializer with the most parameters.

So let’s see how this might look within the implementation of a class:

Notice how starting with init , it calls the next initializer in the chain, initWithStr , passing in a default value (in this case, nil). This method then calls the designated initializer, again, passing in a default value, this time for the date. And notice how the designated initializer is the only method that calls super .

Working with multiple initializers is a simple process of ensuring that each initializer calls up through the initialization chain within the class. This ensures that all instance variables are initialized in just one place, and that the super method is called such that all instance variables of classes further up the hierarchy are initialized first (from top down).

I’ve been spending time getting familiar with Xcode as I learn to write applications in Objective-C, with the larger goal of writing applications for iPhone. My editor of choice on the Mac up to this point, is TextMate, an great all around code editor. I’ve written a number of tutorials/tips on TextMate on this blog (check out the TextMate Category for links).

One of the things I’ve become quite accustomed to is tabbed based user interfaces. For example, in FireFox I typically have a screenful of open tabs. When working with projects inside of TextMate, you can have multiple source code files open in tabs. Unfortunately, this is not a feature supported in Xcode (that I am aware of).

So, I spent a few minutes poking around at various options and came up with an approach that is far from perfect, yet offers a blend of coding in TextMate and managing compiles/builds in Xcode. Not perfect, however, it’s working for me.

Click on the image below to learn more.

Note: What follows is another post in an on-going series for developers who are interested in learning to write applications for the iPhone. More information about writing for the iPhone can be found here:iPhone Developer Tips .

First off, it’s worthing clarifying that much of what is covered here is as much Cocoa as it is Objective-C. However, since the two are quite closely tied when it comes to developing Mac/iPhone applications, I’ve decided to keep with the Objective-C theme as it relates to the title of this and subsequent posts.

The second example, is if you call an accessor method that returns an object. Assume that the object TestClass below has a getter method that returns a pointer to an instance variable named firstName that is an NSString.

This makes sense if you think about it. The getter simple returns a reference (pointer) to an existing object. Now, if you want to retain (express an interest in the object to keep it around) then you would need to use ‘retain’ to bump the reference count. However, if we simply want a pointer to the object as shown here, we don’t have responsibility to release the object.

Initializing Objects

The order of things happening when initializing an object is of great relevance. For example, look at this block of code:

Seems harmless enough, right? Allocate an object and send a message to initialize the object. Here’s the problem, if the init method returns nil (which all good initializers do upon failure to properly initialize an object), the code on line 5 would pass the test (that is, ptr would not be nil).

In the code above, assuming the init method returns nil upon failure, ptr will be set to nil, and the code on line 4 would not pass the test. Much better.

So, the lesson here is twofold: always return nil from an initializer method when the method fails to properly initialize the object. Second, it’s a good practice to combine a call to alloc and init into one step. We’ll talk more about initializers in a separate post.

Releasing Objects

Let’s talk about the other end, releasing objects. Have a look at the implementation for a simple class:

The dealloc method in TestClass releases the memory for the ‘str’ instance variable, and it seems all is well. However, if at some point, somewhere inside the class implementation an attempt was made to check if the ‘str’ instance variable is nil (prior to doing some operation) for example:

if (str)
do something...
else
do something else...

the if statement would pass and the code on line 2 would be run. Not a good thing.

A better way is to replace the call to release with a call to the setter method and pass in nil . Look at the dealloc method below:

The reason this works is that you can send a message to a nil object without raising an exception. If you follow the trail you’ll see inside the setStr method that the variable ‘input’ (which is nil) is sent a message to retain . This is effectively ignored, as the object is nil . The next line releases the string ‘str’. The last line sets ‘str’ to the value of the input parameter (which is nil ). This prevents the problem in the previous example where ‘str’ still had a reference to a location in memory (even though the memory had been released).

As I was getting familiar with this, I wrote a short example in Xcode. Here is the test block for the dealloc method:

-(void) dealloc
{
NSLog(@"Address of 'str' before release is: %ld", str);
// After this, 'str' still points to a memory location[str release];
// After this, 'str' will be nil (** This is the preferred approach **)//[self setStr:nil];
NSLog(@"Address of 'str' is after release is: %ld", str);
// Notice the result of this based on how the instance var 'str' is released (above)if(str !=nil)printf("'str' still points to a memory location (after being released)!");
elseprintf("'str' now points to nil.");
[super dealloc];
}

Notice in this case that line 6 uses release. See the output below:

Now, if you comment out line 6 and remove the comment from line 9, you’ll get the following output. Notice how the reference to ‘str’ after is 0 (nil).

You can download the Xcode project here if you’d like to take a closer look at the complete example. Hopefully this information will save you some time trying to track down an elusive memory leak.

Note: What follows is another post in an on-going series for developers who are interested in learning to write applications for the iPhone. The entire series can be found here:iPhone Developer .

This post introduces messaging within Objective-C. Messaging is the terminology for invoking methods on an object. The format for a message expression is as follows (the brackets are required):

If we want to pass an argument as part of the message (that is, pass a parameter to the method), we send a message that looks like this:

[receiver message:argument]

For example, assume setStr and setX are two methods in the SomeClass object and ptr is a pointer an instance of SomeClass . Here is how we might pass arguments to each method.

[ptr setStr:@"Testing"];
[ptr setX:2008];

Side note: The @ symbol at the front of @”Testing” string is convenience method that converts the given string to an NSString object, which in the case of the setStr method, is the required type for the parameter.

Nest Messages
We can also nest messages, as shown below. In this example, we first pass a message to the NSDate object. This is a class object with a factory method ‘date’ for creating a new NSDate object. If that makes no sense, don’t worry about it. Think of it as nothing more than creating a new NSDate object that holds the current date and time. The return value of inner message (the NSDate object) is the argument for setDate message. What we’ve done here is to create a new date object and pass it as an argument to the setDate message of the receiver (ptr ). Sounds more confusing than it is.

[ptr setDate:[NSDate date]];

Multiple Arguments
Taking this another step further, we can pass multiple arguments along with our message. The message below takes three arguments, a string, date and integer. Sometimes it easier to read the method aloud to get this jist of what’s up. In this case, “pass a message to the ptr receiver that sets a string, and a date and an integer.”

Let me point out a few things about this method. First, notice how in the definition I used the names str , date and x . However, in the implementation, I used the names strInput , dateInput and xInput . The first thing to understand is that you can do this. The reason for doing so, is that in the class where setStr() is defined, I have instance variables with the names str , date and x . If I didn’t change the names in the actual implementation, I’d get a compiler warning that a local declaration hides an instance variable.

In this code example, the simplest thing to do is to change the variable names in the method to make it obvious what variables are referring to what. This may not be the best naming scheme for your applications, however, I wanted to point out the flexibility as to how you define and implement methods and their arguments. Obviously, do what makes the most sense for your specific needs and at the same time, results in the most readable code.

Also, note that this version of setStr() does nothing more than send a message as follows: the receiver is current object instance (self), the message is a setter method and the argument to each message is the appropriate parameter passed in to setStr() .

Using the format/syntax of Objective-C takes some getting used to. However, once you do, you’ll find that it’s easy to read code that has had some thought put into the names of message and the parameters.

Variable Number of Arguments
The last topic is creating messages with a variable number of arguments. Let’s start with the interface definition of a method that accepts a variable number of arguments:

-(void) printInstanceVars:(id)input, ...;

In this example, I declare on parameter that is of type ‘id’, which can represent any object. The implementation of the method looks as follows:

This method will print out each argument using NSLog(). The assumption is made that the last argument will be nil. Here is how you might call the method, passing in two objects. What’s happening here is that I am sending getter messages, if you will, to the ‘ptr’ receiver. Each of these getters returns an object. Notice the nil value as the last parameter.

[ptr printInstanceVars:[ptr str], [ptr date], nil];

Now, this example is a little contrived in that there is probably little value in passing in instance variables from an object, as I’ve done here. However, introducing more classes at this point may confuse more than help. If nothing else, hopefully you get the jist of how this works.

I haven’t come upon the case where this is necessary in an Objective-C application, however, when programming in C, this was quite handy when reading command line parameters. So,if you ever find the opportunity, now you’ll know how. And if you are into trivia, variable argument methods in Objective-C are referred to as variadic methods, go figure.

Many of the things that I’ve covered here are much better understood by looking at a real example. I’ve attached the Xcode project that I created to learn this stuff. You best bet is to download, open the project and mess around with the code.

Here is the main method of the example, so you can get an idea of how I tested each of the above examples.

Download the Xcode Project
Download the Xcode project and take some time to tinker.

Once you’ve got a good handle on the overall application, look at line 27 above. Make any sense? Here’s the deal: the variable length method is expecting objects to be passed in. The code in line 26 will generate a runtime error given the message [ptr x] returns an integer (not an object). The way around this is to create an NSNumber object by sending a message to the NSNumber receiver, with the message ‘numberWithInt’ passing in as the parameter the integer returned from the getter message sent to the ‘ptr’ receiver. If all that makes sense, you’re well on your way :)

For completeness, the entire code listing and a screenshot are shown below:

Note: This post is the start of an on-going series for developers who are interested in learning to write applications for the iPhone. The entire series can be found here:iPhone Developer .

One of the first topics to cover when learning to develop native iPhone applications is how to code in Objective-C. Apple offers the Objective C Reference , a good resource, however, the best way to learn is by writing code. I took to Xcode to write a few simple examples, you’ll find the code below. At the end of this post I also include a link to download the Xcode project I was working with.

There are two aspects to a class, the interface and the implementation, both of which I recommend you store in separate files (although this is not a requirement).

A good coding practice is to save the implementation definition in a file with a name that matches the class name, with an extension of .h (for exampe: SomeClass.h).

This class is inherited from NSObject, the uber object. The class has three instance variables, two that point to other objects, one that references an integer variable. Take note of the getter methods: in Objective-C there is typically no ‘get’ in the front of the method name (in Java this might look like getX or getStr). Second, it should be obvious, that an instance variable can have the same name as a method, as it generally does with a getter. The ‘-’ in the front of the definition, signifies that the method is an instance method. We use a ‘+’ to define a class method (more on class methods in a future post).

One important thing to point out is the format used when declaring methods. For example, setStr() is defined as -(void)setStr: (NSString *) input ; This is translated to, the method setStr is an instance method (given the ‘-’) that returns a void type. The method takes one argument, that is a pointer to an NSString object, the name assigned to the parameter is ‘input’. The reason for the name will become more apparent when you see the implementation of the method below.

Other than learning the syntax of Objective-C, if you are familiar with OO development, most of this should be pretty clear.

A couple of things to point out:

The preferred file name for the implementation is the class name with a .m extension, in this example: SomeClass.m

Notice how this file imports “SomeClass.h” to read the class definition. If you are familiar with C, this is analgous to the #include directive. The benefit of #import is that the compiler will do the work for you to verify that the include file is only read once. If you’ve done any amount of coding in C, you’ll appreciate this convenience, if not, you won’t understand how nice a feature this is.

Within an instance method, all instance variables are within scope. For example, notice how the getter and setter methods refer to the instance variables.

Notice in printInstanceVars() method that there are two means to access the instance variables. You can use the ‘self’ object an send a message to the getter method (more on objects and messages in the next post), or you can directly access the instance variables.

If instance variables are pointers to objects, as are ‘str’ and ‘date’, it’s your responsibility as the developer to free the memory for those objects. The dealloc method is where you do this work. More on that to come…

To complete the example, the code that follows declares an instance of the SomeClass object, and uses the setter/getter methods to print the instance variables to the console.

Let’s go with that for today. In the next post I’ll talk further about this simple example, including instantiation of classes, sending messages to methods and freeing memory of the instance variables.

Having spent the past 8 years working with mobile application development (beginning with the book Core J2ME that I wrote in 2000), it’s odd that its taken me so long to dive into developing for the iPhone. The good news is, I’m now fully engaged and plan to spend a fair amount of time writing about this new endeavor…

This is the first in what I anticipate to be a long series of posts as I ramp up on all things iPhone. The intention is to share how I am going about learning to develop iPhone applications, in the hopes it can help you.

To begin, my plan is to learn the following, in the order shown below (with Xcode, Apple’s development environment, thrown in as needed):

As you’ll see once you download a few of the docs, there is huge amount of information here. To keep things in perspective, my intention is not to revisit all the material in the documents, as much as point out the nuances that I think will be important in becoming a proficient iPhone developer.

A good example is the next post in this series where I’ll point out how to work around one of the sore spots for many who come to Objective-C from an object-oriented language, lack of support for private methods. With Objective-C there are a few tricks to "hide" methods, however, it’s really just a slight-of-hand, so to speak. I’ll explain more later.

There is a fair amount of information to digest above (assuming you are new to Objective-C, Cocoa and/or Xcode). Once you are ready for more, here is a list of additional documents to further immerse yourself:

I hope you’ll join me as learn the ropes for developing iPhone applications.

I was contacted by VisionMobile, a market analysis and strategic advisory firm in the wireless sector, to pass along a link to their most recent survey for developers who are building mobile applications.

Although this is not a Mac tip or trick, I know there is a great deal of application development done on a Mac for a range of mobile devices and platforms. Also, seeing as all developers who complete the survey will be entered into a drawing to win an Amazon voucher worth $1000, I thought you might be interested :)

The survey consists of 5 pages of questions, somewhere around 10-15 minutes of your time. The deadline to complete the survey is Friday, June 27th. The winner will be announced on Friday, July 4th.

If you are working with mobile devices, give it a go, your insight might be helpful to someone else down the road…

I’ve been spending some time with the iPhone SDK to get a perspective on the architecture, tools and overall landscape. In addition, I’ve started to develop a few applications with Xcode (Cocoa/Objective-C) to exercise the tools for building iPhone applications. More on application development next week…

From my perspective, there are two key areas that are exposed. First, the system services in Core OS, accessible through a C library (LibSystem) where one can work with wrappers for low-level features such as threading, networking, file system, Bonjour, among others.

Second, and even more compelling from a user interaction perspective, is the Media layer. Quartz is a 2-D drawing engine. Through the C-based API one can work with vector graphics, lines and shapes, etc. Core Animation is an Objective-C API providing animation and is also part-and-parcel to providing dynamic feedback from the UI. Many of the standard animations that make the iPhone so compelling (screens sliding, flipping over, etc) are included in View classes in the UIKit. OpenGL ES is a mobile version of the OpenGL standard and is the engine behind 3-D graphics. When high-frame rates are in order (think games), OpenGL is the answer.

From what I can tell, the iPhone SDK looks quite comprehensive, providing access across all areas of the device.

Distribution:
One of the challenges since day one for mobile developers has been getting their applications in front of potential users. With the iPhone, applications will be available through Apple’s App Store, which is accessible on the phone as well as through a desktop/laptop system. My understanding is that Apple will offer developers 70% of the revenue. Not unreasonable given Apple will provide the store front and manage all that goes with it.

Overall, an impressive start to what I think could be a significant, and welcome change, to the mobile development landscape.

In a recent email exchange with Allan Odgaard, creator of TextMate, I asked if he had any suggestions for tips that would make for a good screencast. Allan mentioned that on the TextMate IRC, there are often questions about working with comments. And with that, he shared a list of tips that you” onclick=”return TrackClick(”,’javascript%3AlynkVideoPop%28719%2C’)”ll find in the video that follows.

There are five tips on working with comments, including toggling comments on/off, commenting a subset of a line, inserting comment blocks and a short section on how to insert todo lists inside a comment block.